Like pumps for general use, a liquefied gas pump also includes a pump chamber. The pump chamber is formed by a cylinder and a piston, and liquefied gas entrance and discharge ports open into it. The pump chamber is formed by a cylinder and a piston. Relative motion between the piston and the cylinder provides repetition of alternate suction and discharge of liquefied gas through the entrance port into the pump chamber and through the discharge port out of the pump chamber, respectively.
Due to reduction in pressure in the pump chamber during the suction stroke or due to cavitation caused by friction heat generated at surfaces of the piston and the cylinder which slide relative to each other, bubbles of the gas may be generated in the liquefied gas. Such bubbles return to liquid when the pressure of the liquefied gas during the discharge stroke exceeds the saturated vapor pressure of the gas. When the bubbles return to liquid, strong shocks and vibrations could be generated and, at the same time, the metal surfaces of the piston and the cylinder could be eroded.
A pump which can transfer such bubbles out of a pump chamber is shown in, for example, U.S. Pat. No. 5,188,519. In the pump of this patent, bubbles are driven out through the liquefied gas entrance port during suction strokes.
In this pump, the entrance port functions also as a bubble discharge port. It is considered that since bubbles must be discharged through the liquefied gas entrance port against the flow of the liquefied gas, the amount of bubbles which can be discharged out of the pump chamber is relatively small. In particular, since liquefied gas has high viscosity, the fluidity of bubbles is low so that all of them cannot be efficiently discharged.
An object of the present invention is to provide a liquefied gas pump which can drive out bubbles generated in a pump chamber due to cavitation.